This invention relates to a radio transmitter/receiver in which unwanted radiation from a local oscillator does not act as an interferring wave.
In recent years, as the number of mobile radio transmitter/receivers increases, interference is increasingly caused by unwanted radiations from the radio transmitter/receiver. Particularly, the interference due to unwanted radiations from the local oscillator of a transmitter/receiver raises a serious problem because once power is supplied to the radio transmitter/receiver, this type of interferring wave is constantly radiated. For example, in a multi-channel type radio transmitter/receiver in which one channel is used as a control channel and the other channels are used as talking channels, when the control channel is interferred with unwanted radiation of the radio transmitter/receiver of its own system or other system, the performance of the system will be adversely affected. Especially, when the interference is caused by a moving radio transmitter/receiver, it is difficult to find out a source of interference.
Heretofore, in the receivers operating at VHF and UHF bands, intermediate frequencies which are integer multiples of a frequency spacing of 20 KHz or 25 KHz of radio channels of 10.7 MHz, 21.4 MHz, 45 MHz and 70 MHz have been used. The frequencies of the radio channels have also been integer multiples of the frequency spacing. Accordingly, the frequencies of the local oscillators have been integer multiples of the frequency spacings so that when local oscillation waves are radiated from a local oscillator, another radio device would be interferred.